An optical system for a scanner requires the ability to faithfully read the information of the original picture or object being scanned. Accordingly, it is necessary to correct various aberrations like spherical aberration for a single color, as well as to satisfactorily correct axial and transverse chromatic aberration. Axial chromatic aberration increases proportionate to the square of the imaging magnification in an optical system that forms the image of an object arranged at a finite distance, such as in an optical system for a scanner. Thus, the correction of chromatic aberration is even more critical.
Generally, it is necessary with an optical system for a scanner to faithfully reproduce the original picture or object in the visible wavelength region. However, it has also become necessary in recent years for such optical systems to be corrected for chromatic aberration over a wide wavelength range extending from the visible region to the infrared region in the vicinity of 850 nm.
Accordingly, to make such a system apochromatic (i.e., wherein chromatic aberration is corrected over a wide wavelength range), use of anomalous dispersion glass of the phosphoric acid series (e.g., phosphosilicate glass) may be considered. However, in addition to the high cost of the materials for these glasses, there is also the problem of an increase in fabrication costs due to poor workability. Also, temperature changes cause large changes in the refractive index of finished parts made from such glass as compared to ordinary glass, thereby adversely impacting imaging performance. Furthermore, since the refractive index of anomalous dispersion glass is typically low at around 1.5, the number of lenses needed to image with sufficient brightness increases, further increasing cost.
On the other hand, the application of a diffractive optical element (DOE) to an imaging optical system has been proposed for the purpose of correcting chromatic aberration. Due to the deflection of the light by diffraction, the wavelength-dispersion characteristics of a diffractive optical element differs from that of an ordinary refractive optical element. Accordingly, combining a diffractive optical element with a refractive optical element has received attention as a new aberration-correcting means.
The article entitled "The Phase Fresnel Lens," in the Journal of the Optical Society of America, Vol. 51, No. 1, 1961 ("the JOSA reference") discusses a Fresnel lens wherein the phase differential between light passing through adjacent phase rings is 2.pi. with respect to a specified wavelength. The JOSA reference proposes that this phase Fresnel lens is effective as an aberration-correcting means, and discloses an optical system that corrects, for example, spherical aberration by arranging a phase Fresnel lens at the pupil position of the imaging optical system. The Figures show a Schmidt lens and a triplet lens as Working Examples. In addition, the JOSA reference mentions that, taking the wavelength characteristics of the phase Fresnel lens into consideration, it is effective in the correction of the secondary spectrum of a doublet lens. Also, actual design values of a collimator lens are provided. Furthermore, the JOSA reference suggests that axial chromatic aberration can be corrected over a broad wavelength range by arranging a diffractive optical element at the pupil position of the optical system.
Japanese Patent Application Kokai No. Hei 2-1109 discloses an imaging optical system having high resolving power and that corrects spherical aberration and chromatic aberration using a particular type of diffractive optical element called a binary optical element (BOE). In a binary optical element, a step-shaped surface is formed on a light transmitting member using a lithography process. This step-shape surface is capable of partially varying the optical path length. The above-cited Japanese Patent Application discloses also an imaging optical system having a high resolving power and which comprises EL refractive lens element and a transmissive grating element on which is formed a plurality of concentric circular rings. The grating element is arranged at the aperture of the optical system.
However, the invention disclosed in Japanese Patent Application Kokai No. Hei 2-1109 relates to a reduction projection lens for a stepper that uses a KrF excimer laser as the light source. The main objective of the invention is to principally correct spherical aberration to obtain a high-resolution imaging system. The spectral width of the excimer laser is approximately 0.08 nm and chromatic aberration is corrected only over this narrow range. In other words, the disclosed invention uses a limited amount of glass material to correct chromatic aberration in an extremely limited wavelength range of the ultraviolet region, rather than in a wavelength range extending from the visible region to the infrared region.
Japanese Patent Application Kokai No. Hei 8-43767 discloses a photographic optical system for the purpose of correcting the chromatic aberration of a photographic telephoto lens. To correct spot profile (convergence of the ray bundle) as well as distortion and field curvature over the entire image plane, this photographic optical system arranges a diffractive optical element objectwise of a conventional all-refractive telephoto-type optical system. This arrangement corrects characteristic chromatic aberration of the telephoto lens.
Although the invention disclosed in Japanese Patent Application Kokai No. Hei 8-43767 has the objective to correct chromatic aberration over a wide wavelength range when shooting at close range, the field-angle 2.omega. covered is less than 15.degree.. In addition, the imaging optical system disclosed therein arranges all diffractive optical elements most objectwise in the optical system. If an attempt is made to cover a wider field-angle with this configuration, the correction of transverse chromatic aberration becomes problematic even if axial chromatic aberration is corrected. As such, the image quality at the periphery of the field can, no longer be ensured.